Carbohydrate Research 337 (2002) 1229–1233

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Structure of the complex ofheptakis(2,6-di-O-methyl)-�-cyclodextrin with

(2,4-dichlorophenoxy)acetic acid�

Frantzeska Tsorteki, Dimitris Mentzafos*Physics Laboratory, Agricultural Uni�ersity of Athens, Iera Odos 75, GR-11855 Athens, Greece

Received 23 January 2002; received in revised form 18 April 2002; accepted 3 May 2002

Abstract

The structure of the complex formed by heptakis(2,6-di-O-methyl)-�-cyclodextrin and (2,4-dichlorophenoxy)acetic acid wasstudied by X-ray diffraction. The dichlorophenyl moiety of the guest molecule was found outside the host hydrophobic cavity inthe primary methoxy groups region whereas the oxyacetic acid chain penetrates the cavity from the primary face. The hostmolecules stacks along the a crystal axis forming a column. In the space between three successive hosts of the column, a guestmolecule is accommodated. © 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Inclusion compound; Crystal structure; Heptakis(2,6-di-O-methyl)-�-cyclodextrin; (2,4-Dichlorophenoxy)acetic acid; Screw column

1. Introduction

(2,4-Dichlorophenoxy)acetic acid (2,4-D, 2), a syn-thetic compound causing many responses common tonatural auxins, is classified as a plant growth regulatorand not as a hormone, since it is not produced by theplants.2 It is the first selective herbicide discovered and,as it is chemically very stable, it has been widely usedcommercially. Although it exhibits auxin activity at lowconcentrations, as all the phenoxyacetic acids, it be-comes phytotoxic at relatively higher concentrationsand thus it was used as a defoliant in the war inVietnam.3 Heptakis(2,6-di-O-methyl)cyclomaltohep-taose (DM�CD, 1), a chemically modified cyclomalto-heptaose (�-cyclodextrin, �-CD) derivative arising bymethylation of the 2 and 6 hydroxyl groups, exhibits anincreased solubility in water and organic solvents,4

compared to the native �-CD, enabling to use it as acarrier for poorly soluble molecules in hydrophobicsolvents or in water.5,6 The DM�CD molecule is con-

formationally similar to native �-CD as there still existthe hydrogen bonds between the O-2 methoxy and O-3hydroxy groups of adjacent glucose units, keeping themacrocycle rigid. However, steric hindrance due tomethylation of the O-2 hydroxy groups does not allowfor a dimerization, as in the majority of the crystalline�-CD complexes. The methylation extends the depth ofthe hydrophobic torus, estimated to be 10–11 A� com-pared with 7.8 A� for the native �-CD, enabling it toform complexes with larger substrates.7 In spite of thisability of the host, the guest molecules are not foundinside the DM�CD macrocycle cavity in four reportedstructures. In three complexes, those of p-iodophenol/DM�CD, p-nitrophenol/DM�CD8 and 2-naphthoicacid/DM�CD,9 the guest was found outside the cavityin the region of the primary methoxy groups, while incarmofur (1-hexylcarbamoyl-5-fluorouracil)/DM�CD10

it was located in the area of the secondary rim of CD.In three others, those of 1,7-dioxaspiro[5.5]undecane/DM�CD,11 adamantanol/DM�CD7,12 and acetic acid/DM�CD13 complexes, the guest was located inside thecavity. In a continuing effort to study inclusion com-pounds of plant growth regulators in cyclodextrins, weprepared the complex of 2,4-D/DM�CD, the crystalstructure of which is now presented.

� Part III in the series: Inclusion compounds of plantgrowth regulators in cyclodextrins. For Part II, see: Ref. 1.

* Corresponding authorE-mail address: mentz@aua.gr (D. Mentzafos).

0008-6215/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0008 -6215 (02 )00114 -3

F. Tsorteki, D. Mentzafos / Carbohydrate Research 337 (2002) 1229–12331230

2. Experimental

Crystallization.—2,4-D (obtained from Fluka) andDM�CD (purchased from Cyclolab) was dissolved inwater (concentrations 0.02 M) at a 1:1 host:guest moleratio. The resulting mixture was heated at 50 °C andleft at this temperature for a period of three days, at theend of which colorless crystals of the title complexsuitable for X-ray data collection had formed.

X-ray data collection.—Final lattice parameters, de-termined by 32 reflections, are given in Table 1 alongwith other information on data collection and structurerefinement. Data collection was done on a crystal,sealed in a Linderman glass capillary, on a Syntex P21

four-circle diffractometer upgraded by Crystal Logic,attached to a Rigaku rotating anode generator, using agraphite monochromatized Cu K� radiation. Prelimi-nary data collection has indicated the crystal to beorthorhombic. As a consequence, one quarter of thesphere was collected giving 8941 (8245 unique) reflec-

tions with 2�max �100°. The scan mode was �–2�.Three standard reflections monitored every 97 reflec-tions showed an overall decay of 14.2% of the intensity.Lorenz, polarization and decay corrections were ap-plied to the intensity data.

Structure solution and refinement.—The structure wassolved by molecular replacement using the skeletonatom coordinates of the DM�CD molecule of the p-ni-trophenol/DM�CD complex.8 Sequential differenceelectron density maps (��) revealed the positions of theremaining non-hydrogen atoms of the host, all theatoms of the guest and the oxygen atom of one watermolecule. The refinement, based on F2, proceeded byusing the SHELXL-97 program.14 Hydrogen atomslinked to carbon atoms of the host and guest moleculeswere used at calculated positions with C�H distance0.98 A� for the tertiary, 0.97 A� for the secondary, 0.96 A�for the primary and 0.93 A� for the benzyl H-atoms,while their thermal parameters have been set to 1.2×Uiso of the isotropic thermal parameter of the corre-sponding C atom. All the non-hydrogen atoms of thehost molecule, the chlorine atoms of the guest and theoxygen atom of the water molecule were refined an-isotropically. Extinction correction was applied, and 19reflection intensities exhibiting poor agreement weregiven zero weight during the final refinement cycles.The refinement converged at R=0.0753 and 0.1479 forobserved (Io�2.0�(Io)) and all reflections (Table 1),respectively. An atomic numbering of the host andguest molecules is given in Scheme 1. A top and sideviews of the complex are given in Fig. 1. C-mn andO-mn denoting the mth atom within the nth glucosidicresidue of the host.

3. Results and discussion

Molecular geometry and conformation of DM�CD.—Two methyl groups, the primary C-91 and the sec-ondary C-72, and one methoxy group, the O-65�C-95,are disordered over two sites, the occupancies of theirmajor sites being 0.69, 0.66 and 0.54, respectively. TheDM�CD atoms exhibit an increased thermal move-ment, about the same as the atoms of the guestmolecule. This is not unusual, as it is observed also inthe complex of DM�CD with 1,7-dioxaspiro-[5.5]undecane11 and the anhydrous DM�CD.15 Thethermal parameters of C-93 are particularly high butattempts to find more than one atomic sites failed. Fourmethoxy groups have a gauche–gauche orientationpointing outwards of the cavity, see the torsion angles(Table 2). The O-63�C-63 and O-64�C-64 and bothsites of the disordered O-65�C-65 groups exhibit agauche– trans orientation pointing towards theDM�CD cavity. Thus the opening is limited at theprimary side of the CD cavity. The methyl groups of

Table 1Crystal data and structure refinement for 2,4-D/DM�CD

C64H91O35·C8H6O3Cl2·Empirical formula(H2O)0.35

Formula weight 1557.38Temperature (K) 293(2)

1.54180Wavelength, � Cu K� (A� )Crystal system, space group Orthorhombic, P212121

Unit cell dimensions (A� ) a=14.886(7)b=18.980(4)c=28.515(6)

Volume (A� −3) 8057(4)4Z

Dcalcd (Mg/m−3) 1.284Absorption coefficient 1.486

(mm−1)F(000) 3316Crystal size (mm) 0.3×0.4×0.5� Range for data collection 2.80–50.01

(°)Limiting indices (°) −14�h�0

−18�k�5−28�l�28

Reflections collected/unique 8941/8245 [R(int)=0.0414]

50.01, 99.7%Completeness to �

Refinement method Full-matrix least-squares onF2

Data/restraints/parameters 8245/0/888Goodness-of-fit on F2 1.057

R1=0.0753, wR2=0.1900Final R indices [I�2�(I)]R indices (all data) R1=0.1479, wR2=0.2414Absolute structure parameter 0.97 (6)Extinction coefficient 0.00076 (11)Largest diff. peak and hole 0.242 and −0.191

(e A� −3)

F. Tsorteki, D. Mentzafos / Carbohydrate Research 337 (2002) 1229–1233 1231

Scheme 1. An atomic numbering of the host and guest molecules.

the secondary side are oriented away from the torus, anarrangement favorable for the intramolecular O-3n�H···O-2(n+1) hydrogen bonds12 varying between2.754 and 2.972 A� and giving the rigid shape to thetruncated cone of the host. The distances of the glyco-sidic O-4n atoms from their mean plane range between−0.173(6) and 0.134(5) A� , their values being signifi-cantly greater than the observed in �-CD.1,16 The O-4n···O-4(n+1) distances vary between 4.36 and 4.44 A� ,the O-4(n−1)···O-4n···O-4(n+1) angles range from126.3 to 132.2°, while the distances of the approximatecenter K of the O-4n heptagon vary between 4.91 and5.19 A� and the O-4n···K···O-4(n+1) angles range from50.2 to 52.5°. These values indicate that although theO-4n atoms mean plane of the DM�CD molecule ismore puckered than in the native �-CD, its shape isnearly round. The tilt angles of glycopyranose residues,defined as the angles between the O-4n mean plane andthe individual mean planes formed by the O-4(n−1),C-1n, C-4n and O-4n atoms, range between 1.2 (9) and23.5 (7)°, being all positive as in the p-iodophenol andp-nitrophenol/DM�CD isomorphous complexes8 andin two DM�CD-hydrates.17 Therefore, the glucose unitsare tilted towards the approximate sevenfold molecularaxis.

The guest molecules.—The main feature of the struc-ture of the 2,4-D/DM�CD complex is that thedichlorophenyl group of the 2,4-D molecule is locatedoutside the hydrophobic cavity at the primary side ofthe host and the hydrophilic moiety, the oxyaceticgroup, penetrates into the cavity from the primary face(Fig. 1). This arrangement may be attributed to thedifficulty of the aromatic ring, substituted at 2 and 4positions by chlorine atoms, to fit into the rigid cavityand, therefore, staying in the space between three hostmolecules. This arrangement, common to all three crys-tal structures that are isomorphous to the title complex,

is not quite well understood.8,9 An explanation is givenby Harata pretending that, because of the many methylgroups of the hydrophobic cavity of the DM�CDmolecule, unlike that of the native �-CD, a guestmolecule can favorably be accommodated in the inter-stitial site when its shape and size are well suited to thespace.18 Besides, the guest molecules of all these com-plexes have a common feature: they consist of a planarring with hydrophilic moieties bonded to it. Note thatthe adamantol molecule, having a ball shape, is quiteincluded inside the DM�CD cavity.7,11 The characteri-zation of these molecules, not inserting inside theDM�CD cavity, as ‘guests’ is valid only if we acceptthat a guest must not be necessarily fully trapped insidethe cavity to be designated as such.

A hydrogen bond is formed between the O-1 atom ofthe carboxyl group of the guest and the water molecule(2.78 A� ), found inside the cavity and having the lowoccupancy factor of 0.35, but between the host andguest atoms only van der Waals contacts exist. In thep-iodophenol and the p-nitrophenol/DM�CD com-plexes,8 H-bonds are observed between hosts and guestsof the same asymmetric unit while in the 2-naphthoicacid/DM�CD complex, where the carboxyl group ispointing away from the host molecule, such a H-bondis formed between the guest and an adjacent hostmolecule.9 The molecular geometry of 2,4-D is nearlythe expected, the values of the bond lengths and anglesnot differing significantly from the generally accepted.The dichlorophenyl group, the chlorine atoms and theether oxygen atom lie on a plane within 0.038 (17) A� ;the carboxyl group is nearly perpendicular to this planeforming an angle of 75 (1)° with it.

The molecular packing.—The complexes are stackedalong the a crystal axis and the O-4n best planes of thehost molecule forms an angle of 25.4° with the bc plane.The O-4n best planes of two adjacent DM�CD

F. Tsorteki, D. Mentzafos / Carbohydrate Research 337 (2002) 1229–12331232

Fig. 1. A front- and side-view of the complex. C-mn andO-mn denote the mth atom within the nth glycoside residue.

molecules of the same stack form an angle of 50.8°, twoprimary methoxy groups of one DM�CD moleculeentering into the cavity of the other (Fig. 2). As a resultthe DM�CD cavity is closed from the secondary side,the host and guest molecules forming a compactcolumn. The molecular packing consists of suchcolumns linked by the b and c screw axis through vander Waals contacts, and it is characterized as ‘cagetype’,8,9 but it may be designated also as a ‘screwcolumn’, as the host molecules of these compactcolumns are linked by a screw axis.

This molecular packing is found also in the carmo-fur/DM�CD complex10 and the anhydrous DM�CDstructure.15 In the former complex, the fluorouracilmoiety of the guest molecule, disordered over two sites,is located outside the cavity, in the region of thesecondary side of the host. The long alkyl chain of thehexyl group of the major site penetrates into the hostcavity, ending in the primary methoxy groups region,while this of the minor site was found completelyoutside the cavity. The orientation of the guestmolecule is quite different from the observed one in thetitle complex, but at any rate the fluorouracil moietydoes not enter in the cavity. The molecular packing ofthe complexes of DM�CD with guests having an aro-matic or planar moiety is a ‘screw column’. To ourknowledge, only three DM�CD structures have beenfound till now not having a ‘screw column’ molecularpacking: the adamantanol/DM�CD complex where thehosts are stacked along a column,7,12 the 1,7-dioxas-piro[5.5]undecane/DM�CD complex with hosts form-ing layers11 and the acetic acid/DM�CD complex beingof herringbone-type.13 In all those structures, the guestmolecules are entrapped in the cavity and an aromaticmoiety is in any case not involved. The water formsweak hydrogen bonds with the O-64 (2.94 A� ) andO-65A (3.03 A� ) atoms of the host. In the ‘screwcolumn’ structures8–10 the water molecules are alsolocated inside the cavity.

Table 2Selected torsion angles (°) for 2,4-D/DM�CD complex

n=1SiteTorsion n=6 n=7n=5n=4n=3n=2angles

A −97.8 (11) −97.1 (14)C-3n�C-2n�O −111.0 (19) −99.2 (13)−92.0 (14) −97.1 (13) −96.9 (12)B −136.7 (19)-2n�C-7n

142.3 (10)C-1n�C-2n�O A 142.2 (10) 142.1 (13) 125.3 (19) 143.9 (12) 140.3 (12) 140.3 (11)-2n�C-7n 102.5 (19)B

49.4 (12)170.3 (16)−170.8 (16)−160.9 (13) 51.7 (11)46.2 (14)48.7 (13)AC-4n�C-5n�CB −141.7 (18)-6n�O-6nA −68.4 (11) −74.9 (12)O-5n�C-5n�C 76.6 (16) 77.5 (18) 50.1 (19) −72.8 (11) −66.9 (10)

-6n�O-6n 98.1 (19)B−175.7 (10)−171.8 (11)84 (3)−104.1 (19)173 (2)C-5n�C-6n�O −172.5 (11)88.9 (18)A

-6n�C-9n B 126 (2) 176 (4)

F. Tsorteki, D. Mentzafos / Carbohydrate Research 337 (2002) 1229–1233 1233

Fig. 2. A stereo diagram of the channel of the complex. The directions of the axis are indicated.

4. Supplementary material

Full crystallographic details, excluding structure fac-tors, have been deposited with the Cambridge Crystal-lographic Data Center, deposition No CCDC 178967.These data may be obtained, on request, from theCCDC, 12 Union Road, Cambridge CB2 1EZ, UK.Tel.: +44-1223-336408; fax: +44-1223-336033; e-mail:deposit@ccdc.cam.ac.uk.

Acknowledgements

The authors are indebted to Dr. I. M. Mavridis forseveral pertinent comments.

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